Non volatile memory including stabilizing structures

ABSTRACT

An apparatus that includes a magnetic structure including a reference layer; and a free layer; an exchange coupling spacer layer; and a stabilizing layer, wherein the exchange coupling spacer layer is between the magnetic structure and the stabilizing layer and exchange couples the free layer of the magnetic structure to the stabilizing layer.

PRIORITY

This application is a continuation application of U.S. Ser. No.12/502,213 filed Jul. 13, 2009 and which claims priority to U.S.Provisional Application No. 61/117660 entitled “ST RAM WITH EXCHANGECOUPLED SAF STABLE LAYER” filed on Nov. 25, 2008, the disclosures ofwhich are incorporated herein by reference.

BACKGROUND

New types of memory have demonstrated significant potential to competewith commonly utilized types of memory. For example, non-volatilespin-transfer torque random access memory (referred to herein as“STRAM”) and resistive random access memory (referred to herein as“RRAM”) are both considered good candidates for the next generation ofmemory.

In order to reverse a STRAM cell, the spin torque from the writingcurrent has to overcome the in-plane anisotropy (which is equal to 2πMs, where Ms is the saturation magnetization). The in-plane anisotropicfield is about 5000 Oersted (Oe) for a nickel iron (NiFe) layer. Thethermal stability and retention characteristics of such a cell isprovided by the uni-axial anisotropic field, which is only around 500Oe. A memory cell that reduces the in-plane anisotropic field, therebyaffording a cell that is easier to switch, but increases the uni-axialanisotropic field, thereby affording a cell that is more thermallystable and retains the state written to it.

BRIEF SUMMARY

A apparatus that includes a magnetic structure including a referencelayer; and a free layer; an exchange coupling spacer layer; and astabilizing layer, wherein the exchange coupling spacer layer is betweenthe magnetic structure and the stabilizing layer and exchange couplesthe free layer of the magnetic structure to the stabilizing layer.

A non volatile memory cell including a spin torque transfer randomaccess memory (STRAM) structure that includes a reference layer; atunnel barrier; and a recording layer, wherein the tunnel barrier isbetween the reference layer and the recording layer; an exchangecoupling spacer layer; and a stabilizing structure, wherein the exchangecoupling spacer layer is between the STRAM structure and the stabilizingstructure and exchange couples the recording layer of the STRAMstructure to the stabilizing structure.

A method of determining the state of a non volatile memory cellincluding the steps of: providing a non volatile memory cell, the nonvolatile memory cell including a spin torque transfer random accessmemory (STRAM) structure; an exchange coupling spacer layer; and astabilizing structure that includes a synthetic antiferromagnetic (SAF)structure that includes a first ferromagnetic layer, a nonmagneticspacer layer and a second ferromagnetic layer, wherein the nonmagneticspacer layer is between the first ferromagnetic layer and the secondferromagnetic layer; and an upper reference layer, wherein the upperreference layer is proximate to the second ferromagnetic layer,directing a current across the non volatile memory cell, wherein thecurrent is directed from the stabilizing structure to the STRAMstructure; measuring a voltage, wherein the voltage is dependent on themagnetic orientation of the upper reference layer with respect to thesecond ferromagnetic layer of the SAF structure and the orientation ofthe upper reference layer with respect to the second ferromagnetic layercan be changed to reflect one of the two states of the non volatilememory cell.

These and various other features and advantages will be apparent from areading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIGS. 1A and 1B are schematic diagrams of non volatile spin-transfertorque random access memory (STRAM) structures, and FIG. 1C is aschematic diagram of a STRAM cell within a system for utilizing theSTRAM cell as memory;

FIGS. 2A, 2B, and 2C are a schematic diagram defining the relevantdimensions of a STRAM structures (FIG. 2A), a graph showing how thewidth and height affect the effective size of the STRAM structure (FIG.2B), and a graph showing how the driving current is effected by theeffective size (FIG. 2C);

FIG. 3 is a schematic diagram of a STRAM cell that includes anembodiment of a stabilizing structure;

FIGS. 4A through 4E are schematic diagrams of embodiments of astabilizing structure (FIG. 4A), a STRAM cell including such astabilizing structure (FIG. 4B), the STRAM cell of FIG. 4B with theSTRAM structure antiferromagnetically coupled to the stabilizingstructure after a first current has been passed through it in a firstdirection (FIG. 4C), the STRAM cell of FIG. 4B with the STRAM structureferromagnetically coupled to the stabilizing structure after a firstcurrent has been passed through it in a first direction (FIG. 4D), andthe STRAM cell of FIG. 4B with the STRAM structure antiferromagneticallycoupled to the stabilizing structure after a second current has beenpassed through it in a first direction (FIG. 4E);

FIGS. 5A through 5D are schematic diagrams of embodiments of astabilizing structure (FIG. 5A), a STRAM cell including such astabilizing structure (FIG. 5B), the STRAM cell of FIG. 5B with theSTRAM structure antiferromagnetically coupled to the stabilizingstructure after a first current has been passed through it in a firstdirection (FIG. 5C), and the STRAM cell of FIG. 5B with the STRAMstructure ferromagnetically coupled to the stabilizing structure after afirst current has been passed through it in a first direction (FIG. 5D);

FIGS. 6A through 6D are graphs showing the magnetoresistance (Mr) versuscoercivity (H) for a strongly coupled STRAM stack and stabilizingstructure (FIG. 6A) and an intermediately coupled STRAM stack andstabilizing structure (FIG. 6B), the magnetoresistance (also referred toas “MR”) (Mr) versus switching current (I) for a strongly coupled STRAMstack and stabilizing structure (FIG. 6C) and an intermediately coupledSTRAM stack and stabilizing structure (FIG. 6D); and

FIGS. 7A and 7B are graphs showing the magnetoresistance versuscoercivity (FIG. 7A) and magnetoresistance versus switching current(FIG. 7B) of thin recording layers.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, and 5) and any range within that range.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

Non volatile memory cells as disclosed herein include stabilizingstructures. The stabilizing structures generally function to increasethe thermal stability or other characteristics of the non volatilememory cells while still allowing the non volatile memory cells to havean aspect ratio of close to about 1 (i.e. substantially circular insteadof elliptical). The stabilizing structure can utilize different methodsand materials to accomplish this improvement.

A non volatile memory cell utilized herein can include many differenttypes of memory. Exemplary types of memory that can be utilized indevices disclosed herein include, but are not limited to non volatilememory such as, resistive sense memory (RSM). RSM is memory that haschangeable resistance that affords data storage using differentresistance states of the RSM. Exemplary types of RSM include, but arenot limited to, magnetoresistive RAM (MRAM); resistive RAM (RRAM); andspin torque transfer RAM, which is also referred to as STRAM.

In embodiments, the RSM can include STRAM. STRAM includes a MTJ(magnetic tunnel junction), which generally includes two magneticelectrode layers separated by a thin insulating layer, which is alsoknown as a tunnel barrier. An embodiment of a MTJ is depicted in FIG.1A. The MTJ 100 in FIG. 1A includes a first magnetic layer 130,which canalso be referred to as a pinned layer or reference layer, and a secondmagnetic layer 110, which can also be referred to as a free layer or arecording layer. The reference layer 130 and the recording layer 110 areseparated by an insulating layer 120. FIG. 1B depicts a MTJ 100 incontact with a first electrode layer 140 and a second electrode layer151. The first electrode layer 140 and the second electrode layer 151electrically connect the reference layer 130 and the recording layer 110respectively to a control circuit (not shown) providing read and writecurrents through the magnetic layers. The relative orientation of themagnetization vectors of the reference layer 130 and the recording layer110 can be determined by the resistance across the MTJ 100; and theresistance across the MTJ 100 can be determined by the relativeorientation of the magnetization vectors of the reference layer 130 andthe recording layer 110.

The reference layer 130 and the recording layer 110 are generally madeof ferromagnetic alloys such as iron (Fe), cobalt (Co), and nickel (Ni)alloys. Pinning of the reference layer 130 may be achieved through,e.g., the use of exchange bias with an antiferromagnetically orderedmaterial such as PtMn, IrMn and others. The insulating layer 120 isgenerally made of an insulating material such as aluminum oxide (Al₂O₃)or magnesium oxide (MgO).

In embodiments, the recording layer 110 can be a relatively thinrecording layer. In embodiments, the recording layer 110 can be fromabout 1 nm to about 5 nm thick. In embodiments, the recording layer 110can be from about 1 nm to about 3 nm thick. In embodiments, therecording layer 110 can have a relatively high spin polarization. Inembodiments, the recording layer 110 can have a spin polarization thatis equal to or greater than about 0.5. In embodiments, the recordinglayer 110 can generally be made of ferromagnetic alloys such as iron(Fe), cobalt (Co), and nickel (Ni) alloys.

FIG. 1C illustrates a memory device that includes a memory element 113that can include a memory cell 111 and its corresponding transistor 115.Single memory elements 113 can be configured within larger systems. Thememory element 113 can be operatively coupled between a bit line 121 anda source line 125 within a larger system. The read/write circuitry 135controls the particular bit line 121 and source line 125 that currentpasses through, thereby controlling the particular memory cell that isread from or written to. The read/write circuitry 135 can also controlthe voltage applied across the bit line 121 or memory element 113 fromthe source line 125 (or vice versa). The direction which current flowsacross a memory cell 111 is determined by the voltage differentialacross the bit line 121 and the source line 125.

A particular memory cell 111 can be read from by activating itscorresponding transistor 115, which when turned on, allows current toflow from the bit line 121 through the memory cell 111 to the sourceline 125 (or vice versa). The transistor 115 is activated anddeactivated through the word line 131. The word line 131 is operativelycoupled to and supplies a voltage to the transistor 115 to turn thetransistor on so that current can flow to the memory cell 111. Avoltage, dependent on the resistance of the memory cell 111 is thendetected by the sense amplifier 141 from the source line 125 (forexample). The voltage differential between the bit line 121 and thesource line 125 (or vice versa), which is indicative of the resistanceof the memory cell 111 is then compared to a reference voltage 145 andamplified by the sense amplifier 141 to determine whether the memorycell 111 contains a “1” or a “0”.

In order to affect (either read from or write to) a memory cell, acurrent is passed through the STRAM stack perpendicular to the stack. Bychanging the direction of the current, the direction of the recordinglayer can be set either parallel or anti-parallel to the referencelayer. In a STRAM cell with an in-plane recording layer the spin torqueneeds to overcome the in-plane anisotropy of the recording layer inorder to affect the magnetization of the recording layer. The currentnecessary to do this (J_(c0)) can be seen in Equation I below.

$\begin{matrix}{J_{c\; 0} = {\frac{1}{\eta}\left( \frac{2e}{\hslash} \right)\alpha \; M\; {t\left( {H_{K} + {2\pi \; M} + H} \right)}}} & \left( {{Equation}\mspace{14mu} I} \right)\end{matrix}$

In Equation I, η is the spin polarization efficiency of the current, 2e/h is a constant, α is the damping constant, H_(K) and H are theanisotropy field and external field respectively, and M_(s) is thesaturation magnetization of the recording layer. Generally, the term 2πM reflects the in-plane anisotropy and the term Hk reflects theanisotropy. For a typical recording layer (for example, NiFe or CoFeB)the in-plane anisotropy (2 πM) is much larger than the anisotropy field(Hk). The switching current in such a STRAM cell is mostly dominated bythe in-plane anisotropy (2 πM) while the thermal stability and retentionare mostly dominated by (Hk). Therefore, the large 2 πM term makes theswitching current high but does not substantially improve the thermalstability and retention.

Since the intrinsic anisotropy of NiFe and CoFeB (exemplary STRAMmaterials) is low, STRAM cells are commonly manufactured to have anelliptically shaped recording layer in order to induce sufficientthermal stability, by increasing Hk. FIG. 2A depicts an ellipticallyshaped recording layer 110 of a STRAM cell. The recording layer 110 hasa width (also referred to as “w”) and a height (also referred to as“h”). FIG. 2B shows width and height requirements of thermally stablecells. The data in FIG. 2B assumes that the saturation magnetization(Ms) is about 800 emu/cc (typical for NiFe) and the thickness of therecording layer is 3 nm. The effective size is the square root of(width*height). FIG. 2C shows the dependence of the switching current(J(A/m²))on the effective cell size (nm) for cells having differentsaturation magnetization (Ms) values

From a review of FIGS. 2B and 2C, it can be seen that for smaller STRAMcell sizes, larger aspect ratios are generally utilized to maintainthermal stability. For STRAM cells having smaller saturationmagnetization (Ms), smaller critical switching currents can be achieved.However it is very difficult to meet the requirement of both sufficientthermal stability and low switching currents with a single recordinglayer with low Ms.

Non volatile memory cells disclosed herein provide increased thermalstability while maintaining small cell sizes and more importantly smallaspect ratios. Non volatile memory cells disclosed herein generallyinclude a stabilizing structure that is exchange coupled with therecording layer of the MTJ stack. The materials of the stabilizingstructure afford increased thermal stability of the MTJ stack even ataspect ratios approaching 1.

FIG. 3 illustrates embodiments of non volatile memory cells disclosedherein. Generally, non volatile memory cells disclosed herein include aMTJ stack 300, an exchange coupling spacer layer 350, and a stabilizingstructure 340. The exchange coupling spacer layer 350 is generallydisposed between the MTJ stack 300 and the stabilizing structure 340.The MTJ stack 300 includes layers such as those discussed above, areference layer 330, a tunnel barrier structure 320, and a recordinglayer 310. The properties, functions and materials discussed above areapplicable herein. Specifically, the recording layer 310 of the MTJstack 300 generally has a relatively low saturation magnetization andcan create a relatively large spin polarization in the electricalcurrent used to switch the MTJ.

[33] The exchange coupling spacer layer 350 generally functions tomagnetically couple (either ferromagnetically or antiferromagnetically)the recording layer 310 and the stabilizing structure 340. The exchangecoupling spacer layer 350 can be made of various materials, includingbut not limited to, conductive metals such as copper (Cu), tantalum(Ta), ruthenium (Ru), palladium (Pd), platinum (Pt), chromium (Cr), gold(Au), and the like; thin layers of oxide materials such as magnesiumoxide (MgO), alumina (AlO), titanium oxide (TiO), tantalum oxide (TaO),and the like; or combinations thereof. Generally, the exchange couplingspacer layer 350 can be about 5 nanometers (nm) thick if it is made of aconductive metal and about 1 nm thick if made of an oxide material.

Generally, the magnitude of the exchange coupling between the recordinglayer 310 and the stabilizing structure 340 can be at least partiallycontrolled by the materials and thickness of the exchange couplingspacer layer 350. In embodiments, the coupling field between therecording layer 310 and the stabilizing structure 340 will notsignificantly affect the critical current density for switching. Inembodiments, the coupling field can reverse the stabilizing structurewith or without the aid of the static field from the recording layer310. In embodiments, the magnitude of the coupling between the recordinglayer 310 and the stabilizing structure 340 can be in the range of about50 to about 500 Oe.

Generally, exchange coupling can be considered to be strong exchangecoupling if the exchange field (Hex) is equal to or greater than about1000 Oe. An exchange coupling can be considered to be an intermediateexchange coupling if the exchange field (Hex) is from about 100 Oe toabout 1000 Oe.

Embodiments can include a recording layer 310 that creates a relativelylarge spin polarization in the electrical current and also has arelatively low net magnetic moment (Ms*t). The recording layer 310 isexchange coupled (either ferromagnetically or antiferromagnetically) tothe stabilizing layer 340. The exchange coupling can be intermediate, inan embodiment from about 50 to 500 Oe. Therefore, the switching currentwill not be detrimentally affected. The exchange coupling can serve toincrease the total thermal barrier (KuV/kT). In embodiments, the thermalbarrier factor can be increased from about 20 to 30 (without thestabilizing layer) to about 40 or more (with the stabilizing layer). Inembodiments where the thermal barrier factor is increased, for exampleto 40 or more, the recording layer will be more stable (i.e., notsuper-paramagnetic) at room temperature. Because the net moment of therecording layer is small, a substantial reduction in switching currentcan be achieved.

The stabilizing structure 340 generally has a high thermal stability,even when formed in a circular shape (e.g. aspect ratio equals 1). Thestabilizing structure 340 is generally exchange coupled to the recordinglayer 310. The stabilizing structure 340 can include a single layer ormore than one layer. In embodiments, the stabilizing layer can includeferromagnetic layers having acceptable anisotropy. In embodiments, anacceptable anisotropy (Hk) for the stabilizing layer can be greater thanor equal to about 300 Oe. In embodiments, the stabilizing structure 740can generally be made of ferromagnetic alloys such as iron (Fe), cobalt(Co), and nickel (Ni) alloys, including but not limited to CoCr, CoPt,FePt, CoCrPt, and the like. In embodiments, the stabilizing structurecan have a thickness from about 2 nm to about 20 nm.

In embodiments, the stabilizing structure 340 can include syntheticantiferromagnetic (SAF) materials or a SAF structure. SAF structuresgenerally include two or more ferromagnetic layers separated by anonmagnetic layer (or more than one nonmagnetic layer in the case ofmore than two ferromagnetic layers). The ferromagnetic layers areantiferromagnetically coupled, which provides the SAF structures with arelatively high thermal stability. SAF structures also generally havelarge intrinsic anisotropy and do not require any shape inducedanisotropy.

FIG. 4A shows an example of a stabilizing structure 440 that includes aSAF structure. The stabilizing structure 440 includes a firstferromagnetic layer 442, a nonmagnetic spacer layer 444 and a secondferromagnetic layer 446. As seen by the arrows on the firstferromagnetic layer 442 and the second ferromagnetic layer 446, the twolayers are antiferromagnetically coupled. The magnetic orientation ofthe SAF structure (i.e. the three layer structure seen in FIG. 4A) canbe affected by an external magnetic field. For example, a magnetic fieldcan affect the first ferromagnetic layer 442 thereby reorienting themagnetic field to the left (the opposite of that seen in FIG. 4A), thiswill then switch the magnetic orientation of the second ferromagneticlayer 446 so that it is oriented to the right (the opposite of that seenin FIG. 4A). The switch of the first ferromagnetic layer 442 can happenbefore, simultaneous with, or substantially simultaneously with theswitch of the second ferromagnetic layer 446.

The SAF structure 440 can be made of any materials that will exhibit theabove discussed characteristics. Exemplary materials for the first andsecond ferromagnetic layers 442 and 446 include, but are not limited to,cobalt (Co), nickel (Ni), iron (Fe), and combinations thereof. Inembodiments, the ferromagnetic layers can include CoFe, NiFe, andcombinations thereof. The first and second ferromagnetic layers 442 and446 can generally have thicknesses from about 1.5 nm to about 5 nm.Exemplary materials for the nonmagnetic spacer layer 444 include, butare not limited to, ruthenium (Ru), copper (Cu), rhodium (Rh), iridium(Ir), palladium (Pd), chromium (Cr), and the like, or combinationsthereof. In embodiments, the nonmagnetic spacer layer 444 can include Ruor Cu. The nonmagnetic spacer layer 444 can generally have a thicknessfrom about 0.3 nm to about 3 nm.

FIG. 4B shows a non volatile memory cell that includes a stabilizingstructure 440 as illustrated in the embodiment of FIG. 4A. The exemplarynon volatile memory cell includes a MTJ stack 400 having a referencelayer 430, a tunnel barrier structure 420 and a recording layer 410. TheMTJ stack 400 is separated from the stabilizing structure 440 by anexchange coupling spacer layer 450. This embodiment of a stabilizingstructure 440 includes a first ferromagnetic layer 442 and a secondferromagnetic layer 446 separated by a nonmagnetic spacer layer 444.Exemplary characteristics and materials for the various layers of thenon volatile memory cell can be as discussed above.

FIG. 4C depicts the non volatile memory cell after application of acurrent (J in FIG. 4C) that is sufficient to write to the MTJ stack. Thecurrent is directed from the bottom of the MTJ stack to the top of theMTJ stack (i.e. in the direction from the transistor to the MTJ) inorder to write a “1” (i.e. the high resistance state, wherein therecording layer 410 and the reference layer 430 have oppositepolarities). As seen in FIG. 4C, the current will set the recordinglayer 410 to a magnetization (indicated by the left pointing arrow inthe recording layer 410) that is opposite to that of the reference layer430 (which is pinned). The magnetic orientation of the recording layer410 will affect the first ferromagnetic layer 442 of the stabilizingstructure 440 because of the exchange coupling of the recording layer410 and the stabilizing structure 440. The first ferromagnetic layer 442will then affect the second ferromagnetic layer 446 because of itscoupling so that the second ferromagnetic layer 446 is changed (if thefirst ferromagnetic layer 442 was changed) to be anti-parallel to thefirst ferromagnetic layer 442. The timing of the switching of therecording layer 410, the first ferromagnetic layer 442, and the secondferromagnetic layer 446 are not necessarily sequential, and may indeedoccur at substantially the same time.

Upon application of the current, the first ferromagnetic layer 442 willeither remain antiferromagnetically coupled with the recording layer 410or will switch its magnetization direction so that it becomesantiferromagnetically coupled with the recording layer 410. As seen inFIG. 4C, the first ferromagnetic layer 442 had its magnetizationdirection switched (or was already) so that it is opposite to that ofthe recording layer 410. The stabilizing structure 440 in the nonvolatile memory cell depicted in FIG. 4C is antiferromagneticallycoupled with the recording layer 410.

It will also be understood that the stabilizing structure 440 could beferromagnetically coupled to the recording layer 410. The non volatilememory cell depicted in FIG. 4D illustrates the effect of writing a “1”to a non volatile memory cell that contains a stabilizing structure thatis ferromagnetically coupled to the recording layer. As seen in FIG. 4D,the recording layer 410 will affect the first ferromagnetic layer 442 ofthe stabilizing structure 440 because of the exchange coupling of therecording layer 410 and the stabilizing structure 440. Upon applicationof the current (from the bottom to the top of the MTJ stack), theferromagnetic layer 442 will either remain ferromagnetically coupledwith the recording layer 410 or will switch its magnetization directionso that it becomes ferromagnetically coupled with the recording layer410. As seen in FIG. 4D, the first ferromagnetic layer 442 had itsmagnetization direction switched (or was already) so that it isferromagnetically aligned with that of the recording layer 410.

FIG. 4E illustrates the application of a current to write a “0”. Thecurrent is directed from the top to the bottom of the MTJ stack (i.e. inthe direction from the MTJ to the transistor) in order to write a “0”(i.e. the low resistance state, wherein the recording layer 410 and thereference layer 430 have aligned polarities). As seen in FIG. 4E, thecurrent will set the recording layer 410 to a magnetization (indicatedby the right pointing arrow in the recording layer 410) that is alignedwith that of the reference layer 430. The magnetic orientation of therecording layer 410 will affect the first ferromagnetic layer 442 of thestabilizing structure 440 because of the exchange coupling of therecording layer 410 and the stabilizing structure 440. The embodiment inFIG. 4E is antiferromagnetically coupled so that the first ferromagneticlayer 442 will either remain antiferromagnetically coupled with therecording layer 410 or will switch its magnetization direction so thatit becomes antiferromagnetically coupled with the recording layer 410.As seen in FIG. 4E, the first ferromagnetic layer 442 had itsmagnetization direction switched (or was already) so that it is oppositeto that of the recording layer 410. An embodiment that isferromagnetically coupled (not illustrated herein) would switch thefirst ferromagnetic layer so that its magnetization direction is alignedwith the magnetization direction of the recording layer.

Such an embodiment of a non volatile memory cell can have the exchangecoupling (whether ferromagnetic or antiferromagnetic) between therecording layer and the SAF structure that is mediate, or generally inthe range of from about 50 to 500 Oe (which is much less than half ofthe demagnetizing field of about 5000 Oe). Such an embodiment will nothave a significant effect on the switching current. In embodiments, thesaturation magnetization of the recording layer is relatively low sothat a reduction in switching current can be seen. In embodiments, arelatively low saturation magnetization is generally one that is fromabout 400 emu/cc to about 1300 emu/cc.

Another embodiment of a stabilizing structure 540 is depicted in FIG.5A. This embodiment of a stabilizing structure 540 includes a SAFstructure 542. The SAF structure 542 generally includes two or moreferromagnetic layers (first ferromagnetic layer 541 and secondferromagnetic layer 545) separated by a nonmagnetic layer 543 (or morethan one nonmagnetic layer in the case of more than two ferromagneticlayers). The ferromagnetic layers 541 and 545 are antiferromagneticallycoupled through the nonmagnetic layer 543. This embodiment of astabilizing structure 540 also includes a tunnel barrier spacer layer547 disposed between the SAF structure 542 and an upper reference layer549.

The layers of the SAF structure 542 can include similar materials andhave similar properties to those discussed above with respect to thestabilizing structure 440 in FIG. 4A. The tunnel barrier spacer layer547 is generally a material that transmits a spin polarization across itfrom the upper reference layer 549 to the SAF structure 542 (or viceversa). In embodiments, the tunnel barrier spacer layer 547 can be amaterial that transmits a high spin polarization across it from theupper reference layer 549 to the SAF structure 542 (or vice versa).Examples of such materials include interfaces between conventionalferromagnets (such as Co, Fe, CoFeB, and their alloys) and insulatorssuch as magnesium oxide (MgO), magnesium nitride (Mg_(x)N_(y)), andmagnesium oxynitride. Any other materials or combination of materialshaving a relatively high spin polarization may also be used in thetunnel barrier spacer layer 547.

The upper reference layer 549 is generally a layer whose magnetizationorientation is or can be pinned to a particular orientation. Materialsfor the upper reference layer 549 can include, but are not limited toferromagnetic alloys such as iron (Fe), cobalt (Co), and nickel (Ni)alloys. The upper reference layer 549 can be pinned to a particularorientation as is known to those of skill in the art, including the useof exchange bias with an antiferromagnetically ordered material such asPtMn, IrMn and others. In such embodiments, the upper reference layer549 can therefore include more than one layer.

FIG. 5B depicts a stabilizing structure 540 as exemplified with respectto FIG. 5A in combination with a MTJ stack 500 to form an embodiment ofa non volatile memory cell. This exemplary non volatile memory cellincludes a MTJ stack 500, an exchange coupling spacer layer 550, and astabilizing structure 540. The MTJ stack 500 is exchange coupled to theSAF structure 542 of the stabilizing structure 540 through the exchangecoupling spacer layer 550. The exchange coupling of the SAF structure542 to the recording layer 510 of the MTJ stack 500 can be eitherferromagnetic or antiferromagnetic.

FIG. 5C depicts the non volatile memory cell after application of acurrent (J in FIG. 5C) that is sufficient to write to the MTJ stack. Inthis embodiment, the current is directed from the bottom of the MTJstack to the top of the MTJ stack (i.e. in the direction from thetransistor to the MTJ) in order to write a “1” (i.e. the high resistancestate, wherein the recording layer 510 and the reference layer 530 haveopposite polarities). As seen in FIG. 5C, the current will set therecording layer 510 to a magnetization (indicated by the left pointingarrow in the recording layer 510) that is opposite to that of thereference layer 530. The magnetic orientation of the recording layer 510will affect the first ferromagnetic layer 541 of the stabilizingstructure 540 because of the exchange coupling (in this caseantiferromagnetically coupled) of the recording layer 510 and thestabilizing structure 540. The first ferromagnetic layer 541 will thenaffect the second ferromagnetic layer 545 because of its coupling sothat the second ferromagnetic layer 545 is changed (if the firstferromagnetic layer 541 was changed) to be anti-parallel to the firstferromagnetic layer 541. The timing of the switching of the recordinglayer 510, the first ferromagnetic layer 541, and the secondferromagnetic layer 545 are not necessarily sequential, and may indeedoccur at substantially the same time.

The non volatile memory cell depicted in FIG. 5D illustrates the effectof writing a “1” to a non volatile memory cell that contains astabilizing structure that is ferromagnetically coupled to the recordinglayer. As seen in FIG. 5D, the recording layer 510 will affect the firstferromagnetic layer 541 of the stabilizing structure 540 because of theferromagnetic exchange coupling of the recording layer 510 and thestabilizing structure 540. Upon application of the current (from thebottom to the top of the MTJ stack), the first ferromagnetic layer 541will either remain ferromagnetically coupled with the recording layer510 or will switch its magnetization direction so that it becomesferromagnetically coupled with the recording layer 510. As seen in FIG.5D, the first ferromagnetic layer 541 had its magnetization directionswitched (or was already) so that it is aligned with that of therecording layer 510. The second ferromagnetic layer 545 will then switchto be antiferromagnetically coupled with the first ferromagnetic layer541.

Switching the non volatile memory cells depicted in FIGS. 5C and 5D to“0” can be accomplished by directing a current in the opposite direction(not depicted herein). It should be noted that in both the embodimentsdepicted in FIG. 5C and FIG. 5D (as well as that not depicted—switchingthe cells to “0”), the upper reference layer 549 does not switch becauseit is pinned in a single direction. The magnetization direction of theupper reference layer 549 can be the same or different than themagnetization of the reference layer 530 of the MTJ stack.

Methods of reading a non volatile memory cell, which can also bereferred to as determining the resistance state of the non volatilememory cell can generally include directing a current across the nonvolatile memory cell (in either direction) and then measuring a voltage,which is indicative of the resistance of the non volatile memory cell.The resistance states (in embodiments two, a low resistance state and ahigh resistance state) of the non volatile memory cell can be given datastates (in embodiments the low resistance state is given a “0” and thehigh resistance state is given a “1”).

Because the upper reference layer 549 is pinned, it (as well as thereference layer 530 of the MTJ stack) can be used to read the resistancestate of the non volatile memory cell. The magnetic orientation of theupper reference layer 549, and whether the SAF structure isferromagnetically or antiferromagnetically coupled to the MTJ stack willdictate whether or not the resistance state (either high if the upperreference layer 549 is parallel to the second ferromagnetic layer 545 ofthe SAF structure; or low if the upper reference layer 549 isanti-parallel to the second ferromagnetic layer 545 of the SAFstructure) of this type of read is the same or the opposite to that ofthe resistance state of the MTJ stack. For example, as seen in theembodiment depicted in FIG. 5C, if the resistance state was read usingthe reference layer 530 of the MTJ stack, it would show a highresistance state (because of the opposite alignment of the referencelayer 530 and the recording layer 510), which is generally given a valueof “1”. If this same non volatile memory cell was read using the upperreference layer 549 of the stabilizing structure, it would also show ahigh resistance state (because of the opposite alignment of the upperreference layer 549 and the second ferromagnetic layer 545 of the SAFstructure). Therefore, in this embodiment, the particular configurationof the non volatile memory cell (upper reference layer 549 and referencelayer 530 of the MTJ stack having the same pinned direction; and the SAFstructure and the recording layer 510 being antiferromagneticallycoupled) provides two reading schemes where the resistance states arethe same regardless of which way the cell is read.

For purposes of comparison, the configuration of the non volatile memorycell depicted in FIG. 5D (upper reference layer 549 and reference layer530 of the MTJ stack having the same pinned direction; and the SAFstructure and the recording layer 510 being ferromagnetically coupled)will provide different resistance states depending on which way the cellis read from. Such “opposite reading configurations” can be easilyconsidered and compensated for.

Embodiments that utilize an upper reference layer 549 and a tunnelbarrier spacer layer 547 can enhance the output signal when reading. Theadditional tunnel barrier within the overall structure can serve toincrease the signal because of the additional spin polarization of thecurrent. In such an embodiment, the tunnel junction in the MTJ stack canbe designed to reduce the switching current for writing, and the tunneljunction in the stabilizing structure can be designed to increase thecurrent based on the resistance for reading. Therefore, with two barrierlayers, the non volatile memory cell can be designed to both decreasethe switching current and increase the reading signal. Furthermore, sucha design can further reduce the switching current because of the dualspin-filter effect in embodiments where the coupling between therecording layer and the SAF structure is ferromagnetic, and the upperreference layer and the reference layer within the MTJ stack areparallel. In embodiments where the coupling between the recording layerand the SAF structure is antiferromagnetic, the signal may be reducedbecause the magnetoresistances are opposite and will therefore cancel;in such embodiments, it may be advantageous to have themagnetoresistance and the resistance area (RA) of the MTJ stack smallerthan that of the upper stack (the upper reference layer 549, tunnelbarrier spacer layer 547, and second ferromagnetic layer 545).

FIGS. 6A and 6B show how the magnetoresistance (MR) affects the thermalstability (H) in Oersted for an intermediately coupled recording layerand SAF structure (FIG. 6A) and for a strongly coupled recording layerand SAF structure (FIG. 6B). FIGS. 6C and 6D show how themagnetoresistance (MR) affects the switching current (J) for anintermediately coupled recording layer and SAF structure (FIG. 6C) andfor a strongly coupled recording layer and SAF structure (FIG. 6D). Inthe strongly coupled pair, the SAF becomes unbalanced, leading to lowerthermal stability and higher switching current densities. When thecoupling is intermediate, both higher thermal stability and lowerswitching current densities are achieved.

One method of reducing the switching current of a MTJ stack is to reducethe thickness of the recording layer; however, thinner recording layerscan create detrimental properties, including lower thermal energybarriers. If the thermal energy barrier (KuV/kT) is less than about 30,the recording layer can become super-paramagnetic at room temperature.FIGS. 7A and 7B show resistance versus current (FIG. 7A) and resistanceversus coercivity (FIG. 7B) curves for a STRAM cell having a 17 Angstrom(A) thick CoFeB recording layer. As seen from FIGS. 7A and 7B, the cellwill have a relatively small switching current, but is not verythermally stable because of the super-paramagnetic nature.

Thus, embodiments of NON VOLATILE MEMORY INCLUDING STABILIZINGSTRUCTURES disclosed. The implementations described above and otherimplementations are within the scope of the following claims. Oneskilled in the art will appreciate that the present disclosure can bepracticed with embodiments other than those disclosed. The disclosedembodiments are presented for purposes of illustration and notlimitation, and the present disclosure is limited only by the claimsthat follow.

1. A non volatile memory cell comprising: a magnetic tunnel junction(MTJ) stack comprising: a reference layer; a tunnel barrier; and arecording layer having a thermal stability, wherein the tunnel barrieris between the reference layer and the recording layer; an exchangecoupling spacer layer; and a stabilizing structure, wherein the exchangecoupling spacer layer is between the STRAM structure and the stabilizingstructure and exchange couples the recording layer of the STRAMstructure to the stabilizing structure, thereby increasing the thermalstability of the recording layer relative to a recording layer notexchange coupled to a stabilizing structure.
 2. The non volatile memorycell according to claim 1, wherein the MTJ stack has an aspect ratio ofabout
 1. 3. The non volatile memory cell according to claim 1, whereinthe total thermal barrier factor (KuV/kT) of the recording layer isabout 40 or more.
 4. The non volatile memory cell according to claim 3,wherein the recording layer is more stable at room temperature than arecording layer not exchange coupled to a stabilizing structure.
 5. Thenon volatile memory cell according to claim 1, wherein the exchangecoupling between the recording layer of the STRAM cell and thestabilizing structure has a strength from about 50 to about 500 Oe. 6.The non volatile memory cell according to claim 1, wherein the exchangecoupling spacer layer comprises copper, tantalum, ruthenium, magnesiumoxide, or combinations thereof
 7. The non volatile memory cell accordingto claim 1, wherein the stabilizing structure comprises a syntheticantiferromagnetic (SAF) structure.
 8. The non volatile memory cellaccording to claim 7, wherein the SAF structure comprises a firstferromagnetic layer, a second ferromagnetic layer and a nonmagneticspacer layer between the first ferromagnetic layer and the secondferromagnetic layer.
 9. The non volatile memory cell according to claim8, wherein the first ferromagnetic layer of the SAF structure isantiferromagnetically coupled to the recording layer of the MTJ stack.10. The non volatile memory cell according to claim 8, wherein the firstferromagnetic layer of the SAF structure is ferromagnetically coupled tothe recording layer of the MTJ stack.
 11. The non volatile memory cellaccording to claim 1, wherein the recording layer comprises a materialhaving a saturation magnetization of about 400 emu/cc to about 1300emu/cc.
 12. The non volatile memory cell according to claim 1, whereinthe stabilizing structure further comprises an upper reference layer,wherein the upper reference layer is positioned opposite the MTJ stack.13. The non volatile memory cell according to claim 12 furthercomprising a tunnel barrier spacer layer, wherein the tunnel barrierspacer layer is positioned between the SAF structure and the upperreference layer.
 14. The non volatile memory cell according to claim 13,wherein the tunnel barrier spacer layer comprises magnesium oxide,magnesium nitride, magnesium oxynitride, alumina, tantalum oxide,titanium oxide, or combinations thereof
 15. The non volatile memory cellaccording to claim 12 wherein the upper reference layer and thereference layer of the MTJ stack have parallel magnetic orientations.16. A non volatile memory cell comprising: a magnetic tunnel junction(MTJ) stack comprising: a reference layer; a tunnel barrier; and arecording layer having a thermal stability and comprising a materialhaving a saturation magnetization of about 400 emu/cc to about 1300emu/cc, wherein the tunnel barrier is between the reference layer andthe recording layer; an exchange coupling spacer layer; and astabilizing structure, wherein the exchange coupling spacer layer isbetween the STRAM structure and the stabilizing structure and exchangecouples the recording layer of the STRAM structure to the stabilizingstructure with a strength from about 50 to about 500 Oe, therebyincreasing the thermal stability of the recording layer relative to arecording layer not exchange coupled to a stabilizing structure.
 17. Thenon volatile memory cell according to claim 16, wherein the MTJ stackhas an aspect ratio of about
 1. 18. The non volatile memory cellaccording to claim 16, wherein the total thermal barrier factor (KuV/kT)of the recording layer is about 40 or more.
 19. A non volatile memorycell comprising: a magnetic tunnel junction (MTJ) stack comprising: areference layer; a tunnel barrier; and a recording layer having athermal stability and comprising a material having a saturationmagnetization of about 400 emu/cc to about 1300 emu/cc, wherein thetunnel barrier is between the reference layer and the recording layer;an exchange coupling spacer layer comprising copper, tantalum,ruthenium, magnesium oxide, or combinations thereof; and a stabilizingstructure comprising a synthetic antiferromagnetic (SAF) structure,wherein the exchange coupling spacer layer is between the STRAMstructure and the stabilizing structure and exchange couples therecording layer of the STRAM structure to the stabilizing structure witha strength from about 50 to about 500 Oe, thereby increasing the thermalstability of the recording layer relative to a recording layer notexchange coupled to a stabilizing structure.
 20. The non volatile memorycell according to claim 19, wherein the MTJ stack has an aspect ratio ofabout 1.